It has been extensively argued that the space around us is a sensory-motor interface for interactions with the environment, both for controlling voluntary object-oriented actions and for defending the body against potentially threatening stimuli. Hereafter, I will show that such interactions are not limited to low-level hand-objects contacts, but could also include abstract and cognitive exchanges with other people. Most of the studies previously presented in this dissertation investigated how the representation of the space surrounding the body changes as a function of interaction with an artificial object: by using a tool, for instance, the distinction between a near space - where I can normally act - and the far space – where I can act with the tool - is altered.
Critically, in everyday life the distinction between near and far space is meaningful not only in terms of possibility of object interaction, but mainly in terms of social interactions. That is to say, not only the nature of the action one is performing is important in defining the space immediately adjacent to the body, but also the social and emotional valence of the target as well as the interpersonal consequences of the actions.
Accordingly, a new corpus of research has begun to explore how selectively social information can affect how we represent the space around us. A distinction can be made between studies of the role of PPS during social interaction and those investigating how affective information influences PPS coding. While in the first case several investigations have revealed how PPS could be affected by social factors such as presence, morality, or cooperative behaviour of other individuals; only few studies have determined how the emotional (not strictly social) relevance of the stimuli influenced the layout of PPS. Since this latter topic is less relevant for this dissertation, first I briefly will summarize these emotional-related studies before considering the social-related PPS investigations.
A few studies investigated how (perceived) threat affects sensory processing in PPS. For example, clear inter-individual differences in the extension of the defensive PPS, as indexed by distance dependent modulations of HBR, strongly and positively related to the variability in trait anxiety (Sambo and Iannetti, 2013). Other researchers, meanwhile, have reported that the size of PPS is correlated with the extent of claustrophobia, a condition characterised by intense anxiety in relation to enclosed spaces and physically restrictive situations (Lourenco et al., 2011, see Taffou and Viaud-Delmon, 2014, for the relation between cynophobia and PPS). In Lourenco and colleagues’ work (2011), the effect of claustrophobia on PPS has been explored by measuring spatial biases in visual bisection task: when bisecting horizontal lines close to the body, individuals show a slight leftward bias that, however, shifts rightward when the line is presented in far space
(Longo and Lourenco, 2006, Patanè et al 2015a). The authors found that more claustrophobic subjects showed a more gradual rightward shift over distance, a finding interpreted as evidence that these individuals had a larger representation of their PPS. Although it is important to highlight that the extension of the defensive PPS is expected to vary as a function of the threat context in which it is measured, changes in PPS extension correlate with the anxiety similarly to what observed for claustrophobia: figuratively speaking, the greater the fear or anxiety, the bigger the PPS extent.
Within the same line of research, a recent study investigated whether visual information in PPS could specifically affect processing of nociceptive stimuli and demonstrated that the interaction between visual and nociceptive stimuli also depends on the region of space in which visual information is presented (De Paepe et al., 2014). Related to this, the proximity of threat also seems to affect distance estimation of stimuli relative to the body (see Tabor et al., 2015) and threatening looming stimuli are perceived as having shorter time to- impact latency as compared to nonthreatening objects moving at the same speed (Vagnoni et al., 2012). More interestingly, the distance from the body at which auditory stimuli starts to affect RTs to tactile targets on the hand is also larger for auditory stimuli with negative compared positive valence. This is a result that could be interpreted as an extension of the defensive PPS for threatening or negative stimuli (Ferri et al., 2015b).
More relevant for this dissertation, PPS representation is shaped both by presence and behaviours of other individuals. The pioneering work by Heed and colleagues (2010) showed for the first time that multisensory integration is modulated as a function of the presence and activities of others located within one’s own PPS. In their behavioural study employing the classic CCE, a group of participants had to respond to the elevation (up or down) of tactile stimuli applied on the hand, and to ignore visual distractor presented concurrently in a congruent or incongruent manner compared to the tactile stimulus. Crucially, participants performed the task both alone and with a partner who responded to the visual distractors. CCE was significantly reduced when the participant performed the task with a partner who sat in front to her and concurrently responded to visual stimuli. Note, however, that the social modulation of CCE required the partner's presence and action within the participant's PPS. When the partner was outside of the participant's PPS, or she did not perform a task on visual stimuli, no modulation of CCE was observed. Such a change of CCE was interpreted as due to top-down modulation, so that knowing that the partner acts upon visual events near to one's body reduces the crossmodal interactions between vision and touch in the space around us. In other words, the possibility exists that the PPS may shrink when other agents act into our vicinity.
Teneggi et al. (2013) extended this intriguing result by describing both that PPS is smaller when facing another individual standing in far space and that the PPS boundary changes as a function of the social experience with the other individual. The size of PPS was assessed in terms of the distance at which approaching sounds started to decrease participants’ detection latencies in response to tactile stimuli delivered to the their face. The first experiment showed that PPS representation shrinks (i.e., PPS boundary move closer to the subject' body) when the far space is occupied by another person, as compared to when it is occupied by an artificial, human-like object (i.e. a mannequin) of comparable size. The fact that the boundary of PPS was closer to the participants seems to indicate that the mere presence of an unknown person shapes PPS representations, as if people automatically and implicitly divide the space between themselves and others. Even more interestingly, in a second experiment the size of PPS was found to increase after playing an economic game with a cooperative individual positioned in front of the participant, as compared to before playing the economic game. Critically, this was not the case when the game was played with an uncooperative individual. In other terms, the shift of PPS boundary when the other individual behaved cooperatively was interpreted as an enlargement of one’s own PPS as to include the space around the other. By contrast, when the other individual failed to cooperate during the economic interaction, the PPS boundaries between self and the other did not change. Thus, this study showed that PPS representation not only responds to the presence of others, but is also shaped by interactions with others and, more specifically, by valuation of other people's behaviour during the social interaction. Lastly, as a further demonstration that PPS representation is sensitive to the social perception of the other, in a recent work adapting the same paradigm in mixed realty, PPS was found to be more extended when participants were facing a moral than when facing an immoral person. This effect was specific for social context, as no change in PPS was detected if participants were facing an object, instead of the person (Pellecin et al., 2017, see also Iachini et al., 2015 and 3.4 session of the present dissertation).
Finally, a recent fMRI adaptation study (Brozzoli et al., 2013) explored whether in the human brain a shared representation of PPS for oneself and another person could exists.
Specifically the authors tested if the human brain contains neuronal populations encoding the space near both one's own hand and another person's hand, analogous to what was identified in monkey parietal cortex (Ishida et al., 2010, see previous paragraphs). In this study, participants viewed a ball moving either near to their hand, or to another person's hand, which was positioned with the same orientation of the participants' hand but located in far space. An artificial hand, also located far from the subject, served as a control condition. By employing BOLD adaptation, a pool of neurons was discovered in the left PMv exhibiting mirror-like properties. Namely, a subset of the neuronal
populations that displays selectivity for an object near a person’s own hand also displays the same selectivity for representing an object close to another person’s hand rather than close to the artificial hand. This finding fits well with the discovery in the macaque’s brain of visuo-tactile populations of parietal neurons discharging when the monkey sees an object moving close to another individual’s body (the “body-matching neurons” identified by Ishida and others 2010). This mirror-like, embodied simulation mechanism may provide the neural substrate for how the representation of one's own PPS accommodates in the presence of others during social interactions (Lava das and di Pellegrino, 2015). Consistent with this compelling hypothesis, the last study of this thesis will explore whether a highly social construct might be rooted in such simulative mechanisms.